The long-term objective of this project aims to use the crayfish Y-organ, a simpler model of vertebrate steroid hormone-secreting glands, for wide-ranging studies on control of steroid hormone synthesis and secretion. Basically, it is a problem of peptide hormone regulation of steroidogenesis. Crayfish Y-organs are under direct control by a molt-inhibiting hormone which suppresses synthesis and secretion of the steroid hormone ecdysone. They are easily acessible, they culture easily in vitro and they consist of one cell type. Y-organs extract cholesterol (exdysone precursor) from the hemolymph, they do not significantly esterify sterol, side chain splitting is not required for hormone production and the product is not stored in any appreciable amount. In the long term we seek to provide some understanding of specific mechanisms involved in the process of regulation of ecdysone secretion by the crayfish Y-organ. The specific aims of this application seek an understanding of initial events in the process. Specifically, to demonstrate that molt-inhibiting hormone (MIH) exerts its action through a cyclic nucleotide and, perhaps calcium, second messenger system. We aim to show increased activity in a membrane adenylate cyclase system, regulation by a phosphodiesterase, the role of calcium ion in MIH action, elevated levels of cyclic AMP, possibly, some evidence for subsequent activation of a protein kinase, and a final reduction of ecdysone secretion. The Y-organ will be cultured in a defined medium which will be measured for secreted ecdysone by a radioimmuno assay. Test chemicals will be added to, or subtracted from, the medium. For an individual animal one Y-organ from the pair will serve as control, the other as experimental. Enzymes will be extracted and assayed according to established procedures. This simpler model, crayfish Y-organ, of vertebrate steroid-secreting organs could prove to be useful in understanding the complex pathways from peptide hormone binding at the cell membrane to the secretion of a steroid, which in turn affects other cells, in vertebrates and humans.